FIG. 1 describes a prior art ballast 10 for providing instant start operation of a gas discharge lamp 40. Ballast 10 includes a full-wave rectifier circuit 100, a boost converter 200, a self-oscillating current-fed half-bridge inverter 300, a parallel resonant output circuit 400, and an inverter startup circuit 500.
Before AC power is applied to ballast 10, boost converter 200 and inverter 300 are off. Once AC power is applied, boost converter 200 and inverter are still off, and the DC rail voltage VDC goes from zero to the peak of the voltage provided by AC voltage source 30. At that time, within inverter startup circuit 500, capacitor 540 begins to charge up (via connection point A and input 502) through resistor 510. Eventually, the voltage VX across capacitor 540 reaches the breakover voltage (e.g., 32 volts) of diac 550, at which point diac 550 turns on. When diac 550 turns on, the stored energy in capacitor 540 causes a current pulse to be injected (via output 504 and connection point B) into the base of lower inverter transistor 340, thereby causing inverter 300 to begin to operate. Diode 560 (which is connected to inverter output terminal 306 via output 506 and connection point C) prevents capacitor 540 from charging up and activating diac 550 while inverter 300 is operating.
Boost converter 200 begins to operate once boost control circuit 220 is activated, which (in general) may occur either before or after inverter 300 begins to operate. Once boost converter 200 begins to operate, VDC begins to increase (from the peak of the voltage provided by AC source 30) and eventually reaches its steady-state operating level.
For an instant start ballast, it is highly preferred that boost converter 200 begin to operate prior to startup of inverter 300. More particularly, it is preferred that inverter 300 be started only after VDC is high enough so that inverter 300 and output circuit 400 can provide a ballast output voltage that is sufficiently high to ignite lamp 40 in a preferred manner (i.e., with little or no glow current and a fast strike time).
In inverter startup circuit 500, the time that it takes for VX to reach the diac breakover voltage is a function of the magnitude of the voltage provided by AC source 30, the resistance of resistor 510, and the capacitance of capacitor 540. In theory, resistor 510 and capacitor 540 may be selected so that, for a given AC source voltage, inverter startup is delayed until VDC is at or near its steady-state operating level. Unfortunately, this is not true in practice because the permissible values for resistor 510 and capacitor 540 are heavily constrained by the electrical limitations of diac 550. In particular, the peak current and power ratings of diac 550 dictate that capacitor 540 must be fairly small (e.g., on the order of 0.1 microfarads or so), while the leakage current of diac 550 places an upper limit on resistor 510 (i.e., resistor 510 must be small enough to supply the maximum diac leakage current, as well as additional current for charging up capacitor 540). Thus, in practice, it is generally not possible to select resistor 510 and capacitor 540 so that inverter startup is delayed until VDC is at or near its steady-state operating level.
If the AC source voltage varies over a wide range (as it does in the case of so-called universal input voltage ballasts, wherein the nominal range of the AC source voltage is between 120 volts and 277 volts), the aforementioned difficulties are especially pronounced. For example, even if it were possible to design inverter startup circuit 500 so that lamp 40 receives optimal ignition voltage when the AC source voltage is 277 volts, the same will not occur when the AC source voltage is at 120 volts. Startup circuit 500 is therefore particularly ill-suited for universal input voltage applications.
What is needed, therefore, is a ballast with an inverter startup circuit that provides an appropriate delay period so that the ballast can provide sufficient voltage for igniting a lamp in a preferred manner. Such a ballast and inverter startup circuit would represent a significant advance over the prior art.